1. Field of the Invention
The present invention relates to switching power supplies, and more specifically to an Electro-Magnetic Interference (EMI) reduction circuit for power converters.
2. Description of the Related Art
FIG. 1A shows a schematic of a power converter, which is a PFC (power factor correction) power converter. The power converter as shown in FIG. 1A comprises an EMI filter 5, a bridge rectifier 10, an inductor 11, a rectifier 12, a power switch 13, a bulk capacitor 14, a voltage divider formed by resistors 16 and 17, and a controller 90. The operation of the PFC power converter shown in FIG. 1A is well known to those skilled in the art and will be omitted herein.
In order to meet regulations, such as FCC emission standards for electro-magnetic interference (EMI), the EMI filter 5 is equipped between an alternating current (AC) mains VAC and the bridge rectifier 10. However, the EMI filter 5 occupies significant layout space and increases component costs to power converters. Some prior arts had proposed solutions to eliminate the need of the EMI filter 5, such as U.S. Pat. No. 7,203,079 titled “Switching Controller Having Frequency Hopping for Power Supplies” and U.S. Pat. No. 7,391,628 titled “Switching Controller Having Frequency Hopping for Power Supplies and Method Therefore”. The aforementioned prior arts need a jittering signal generation circuit (pattern generator) which occupies significant space on the controller chip and increases the manufacturing cost. Therefore, a cost-effective frequency jittering circuit is desired by the industry.